For internal combustion engines, such as diesel engines, nitrogen oxides (NOx) compounds may be emitted in the exhaust. To reduce NOx emissions, a selective catalytic reduction (SCR) process may be implemented to convert the NOx compounds into more neutral compounds, such as diatomic nitrogen, water, or carbon dioxide, with the aid of a catalyst and a reductant. The catalyst may be included in a catalyst chamber of an exhaust system, such as that of a vehicle. A reductant, such as anhydrous ammonia, aqueous ammonia, or urea, is typically introduced into the exhaust gas flow prior to the catalyst chamber. To introduce the reductant into the exhaust gas flow for the SCR process, an SCR system may dose or otherwise introduce the reductant through a dosing module that vaporizes or sprays the reductant into an exhaust pipe of the exhaust system upstream of the catalyst chamber.
Prior to being dosed into the exhaust pipe of the exhaust system, the reductant is stored in a tank and engines may monitor the amount of reductant, such as urea or diesel exhaust fluid (DEF) present in the tank. To monitor the amount of stored reductant, a sensor or multiple sensors may be coupled to the tank. In some systems, the stored reductant is also used as coolant for the dosing module. Use of urea or DEF as a coolant requires a reserve amount of the reductant be present in the tank to allow for recirculation. At low tank-level sensor values, dosing must stop to ensure sufficient urea for coolant recirculation, which brings tank-level sensors within the scope of the on-board diagnostic system. Thus, various OBD requirements, including requirements stipulated by the United States Environmental Protection Agency (EPA) and the California Air Resources Board (CARB), mandate diagnosing these tank-level sensors as part of NOx emission reduction control.